1. Field of the Invention
This invention is related to the field of use of image capture. More particularly, this invention is directed to a method and apparatus for determining the nature of the illuminant during still and video image capture.
2. Background
Video and still image capture using a digital camera has become very prevalent. Video capture may be used for such applications as video conferencing, video editing, smart vision and distributed video training. Still image capture with a digital camera may be used for such applications as photo albums, photo editing, and compositing. Many components of hardware and software need to work seamlessly to both get the video data (also referred to as a video “stream”) or the still image data from the hardware through the various layers of software on the PC and made visible on a display medium.
Digital cameras are typically expected to operate under a variety of scene illuminations. Some common illuminant conditions are fluorescent lighting, day light (also known as D65 lighting), and tungsten illumination. These light sources have different spectral (wavelength) components, as shown in FIG. 1. For example, the tungsten light source has a stronger red wavelength component (600 nm to 700 nm) compared to the blue wavelength component (400-480 nm), and green wavelength component (480-580 nm). In comparison, D65 lighting has stronger blue and green wavelength components with a relatively weaker red wavelength component. Imaging under these conditions requires careful exposure settings to establish linear region of operation of the digital camera within the photospace in all color channels. Part of the challenge is in obtaining color balance and accurate reproduction of color and tone under these different illuminating conditions. If the type of illuminant is known, then the correct exposure controls and algorithms can be applied to obtain accurate colors in images.
Currently, many imaging systems rely on pre-metering information and a set of histograms to identify the relative strengths in the red, green and blue channels to identify the nature of the illuminant. This process consumes time and sets a lower bound on the pre-metering time.